Biodegradable polymers having controlled releasing property are useful, for example, as base materials of microcapsules and the like for containing a physiologically active substance. It is known that, as such biodegradable polymers, polymers containing polylactic acid, a copolymer of lactic acid and glycolic acid, and the like are useful (JP-A No. 11-269094 and the like).
As these biodegradable polymers, those produced by a conventional synthesis method have been used themselves, however, it has been found that those produced themselves have poor availability as a controlled release base material since they have low content of end carboxyl groups. Therefore, it has been studied to hydrolyze an biodegradable polymer as described above having higher molecular weight to control the weight-average molecular weight thereof to suitable level, before use as a base material for a controlled release preparation.
However, polymers obtained by hydrolysis and water-washing tend to cause an initial burst of physiologically active substance, rendering them unsuitable as a controlled release base material, even if they have an appropriate weight-average molecular weight and end carboxyl group content. Therefore, an improvement is desired in current conditions.
JP-A No. 7-97334 discloses a controlled release preparation composed of a physiologically active peptide or salt thereof and an biodegradable polymer having a free carboxyl group at the end, and a method of producing the same.
GB2209937, GB2234169, GB2234896, GB2257909 and EP626170A2 disclose a composition containing as a base material a biodegradable polymer containing a water-insoluble salt such as pamoates of peptide and protein prepared separately, and a method of producing this composition.
WO95/15767 discloses an embonate (pamoate) of cetrorelix (LH-RH-antagonist) and a method of producing the same, and simultaneously describes that even if this pamoate is enclosed in an biodegradable polymer, the peptide releasing property thereof is the same as for a pamoate alone.